Cytochrome c oxidase

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Cytochrome c oxidase
296px
The crystal structure of bovine cytochrome c oxidase in a phospholipid bilayer. The intermembrane space lies to top of the image. Adapted from PDB: 1OCC​ (It is a homo dimer in this structure)
Identifiers
EC number 1.9.3.1
CAS number Template:CAS
Databases
IntEnz IntEnz view
BRENDA BRENDA entry
ExPASy NiceZyme view
KEGG KEGG entry
MetaCyc metabolic pathway
PRIAM profile
PDB structures RCSB PDB PDBe PDBsum
Gene Ontology AmiGO / EGO

The enzyme cytochrome c oxidase, or Complex IV (EC 1.9.3.1), is a large transmembrane protein complex found in bacteria and the mitochondrion of eukaryotes.

It is the last enzyme in the respiratory electron transport chain of mitochondria or bacteria located in the mitochondrial or bacterial membrane. It receives an electron from each of four cytochrome c molecules, and transfers them to one oxygen molecule, converting molecular oxygen to two molecules of water. In the process, it binds four protons from the inner aqueous phase to make water, and in addition translocates four protons across the membrane, helping to establish a transmembrane difference of proton electrochemical potential that the ATP synthase then uses to synthesize ATP.[1][2]

Structure

Subunit I and II of Complex IV excluding all other subunits, PDB: 2EIK​

The complex is a large integral membrane protein composed of several metal prosthetic sites and 14 [3] protein subunits in mammals. In mammals, eleven subunits are nuclear in origin, and three are synthesized in the mitochondria. The complex contains two hemes, a cytochrome a and cytochrome a3, and two copper centers, the CuA and CuB centers.[4] In fact, the cytochrome a3 and CuB form a binuclear center that is the site of oxygen reduction. Cytochrome c, which is reduced by the preceding component of the respiratory chain (cytochrome bc1 complex, complex III), docks near the CuA binuclear center and passes an electron to it, being oxidized back to cytochrome c containing Fe3+. The reduced CuA binuclear center now passes an electron on to cytochrome a, which in turn passes an electron on to the cytochrome a3-CuB binuclear center. The two metal ions in this binuclear center are 4.5 Å apart and coordinate a hydroxide ion in the fully oxidized state.

Crystallographic studies of cytochrome c oxidase show an unusual post-translational modification, linking C6 of Tyr(244) and the ε-N of His(240) (bovine enzyme numbering). It plays a vital role in enabling the cytochrome a3- CuB binuclear center to accept four electrons in reducing molecular oxygen to water. The mechanism of reduction was formerly thought to involve a peroxide intermediate, which was believed to lead to superoxide production. However, the currently accepted mechanism involves a rapid four-electron reduction involving immediate oxygen-oxygen bond cleavage, avoiding any intermediate likely to form superoxide.[5]

Assembly

The site of assembly is believed to occur near TOM/TIM, where complex intermediates are accessible to bind with subunits imported from cytosol. Hemes and cofactors are inserted into subunits I & II. Subunits I and IV initiate assembly. Other subunits may form sub-complex intermediates that later on, bind to others sub-complexes to form holo COX. In post-assembly modifications, the enzyme is dimerized, which is required for active/efficient enzyme action.[6][7]

Complex subunits

Conserved subunits of the cytochrome oxidase c complex:[8][9]

No. Subunit name Human protein Protein description from UniProt Pfam family with Human protein
1 Cox1 COX1_HUMAN Cytochrome c oxidase subunit 1 Pfam PF00115
2 Cox2 COX2_HUMAN Cytochrome c oxidase subunit 2 Pfam PF02790, Pfam PF00116
3 Cox3 COX3_HUMAN Cytochrome c oxidase subunit 3 Pfam PF00510
4 Cox4i1 COX41_HUMAN Cytochrome c oxidase subunit 4 isoform 1, mitochondrial Pfam PF02936
5 Cox4a2 COX42_HUMAN Cytochrome c oxidase subunit 4 isoform 2, mitochondrial Pfam PF02936
6 Cox5a COX5A_HUMAN Cytochrome c oxidase subunit 5A, mitochondrial Pfam PF02284
7 Cox5b COX5B_HUMAN Cytochrome c oxidase subunit 5B, mitochondrial Pfam PF01215
8 Cox6a1 CX6A1_HUMAN Cytochrome c oxidase subunit 6A1, mitochondrial Pfam PF02046
9 Cox6a2 CX6A2_HUMAN Cytochrome c oxidase subunit 6A2, mitochondrial Pfam PF02046
10 Cox6b1 CX6B1_HUMAN Cytochrome c oxidase subunit 6B1 Pfam PF02297
11 Cox6b2 CX6B2_HUMAN Cytochrome c oxidase subunit 6B2 Pfam PF02297
12 Cox6c COX6C_HUMAN Cytochrome c oxidase subunit 6C Pfam PF02937
13 Cox7a1 CX7A1_HUMAN Cytochrome c oxidase subunit 7A1, mitochondrial Pfam PF02238
14 Cox7a2 CX7A2_HUMAN Cytochrome c oxidase subunit 7A2, mitochondrial Pfam PF02238
15 Cox7a3 COX7S_HUMAN Putative cytochrome c oxidase subunit 7A3, mitochondrial Pfam PF02238
16 Cox7b COX7B_HUMAN Cytochrome c oxidase subunit 7B, mitochondrial Pfam PF05392
17 Cox7c COX7C_HUMAN Cytochrome c oxidase subunit 7C, mitochondrial Pfam PF02935
18 Cox7r COX7R_HUMAN Cytochrome c oxidase subunit 7A-related protein, mitochondrial Pfam PF02238
19 Cox8a COX8A_HUMAN Cytochrome c oxidase subunit 8A, mitochondrial P Pfam PF02285
20 Cox8c COX8C_HUMAN Cytochrome c oxidase subunit 8C, mitochondrial Pfam PF02285
Assembly subunits[10][11][12]
1 Coa1 COA1_HUMAN Cytochrome c oxidase assembly factor 1 homolog Pfam PF08695
2 Coa3 COA3_HUMAN Cytochrome c oxidase assembly factor 3 homolog, mitochondrial Pfam PF09813
3 Coa4 COA4_HUMAN Cytochrome c oxidase assembly factor 4 homolog, mitochondrial Pfam PF06747
4 Coa5 COA5_HUMAN Cytochrome c oxidase assembly factor 5 Pfam PF10203
5 Coa6 COA6_HUMAN Cytochrome c oxidase assembly factor 6 homolog Pfam PF02297
6 Coa7 COA7_HUMAN Cytochrome c oxidase assembly factor 7, Pfam PF08238
7 Cox11 COX11_HUMAN Cytochrome c oxidase assembly protein COX11 mitochondrial Pfam PF04442
8 Cox14 COX14_HUMAN Cytochrome c oxidase assembly protein Pfam PF14880
9 Cox15 COX15_HUMAN Cytochrome c oxidase assembly protein COX15 homolog Pfam PF02628
10 Cox16 COX16_HUMAN Cytochrome c oxidase assembly protein COX16 homolog mitochondrial Pfam PF14138
11 Cox17 COX17_HUMAN Cytochrome c oxidase copper chaperone Pfam PF05051
12 Cox18[13] COX18_HUMAN Mitochondrial inner membrane protein (Cytochrome c oxidase assembly protein 18) Pfam PF02096
13 Cox19 COX19_HUMAN Cytochrome c oxidase assembly protein Pfam PF06747
14 Cox20 COX20_HUMAN Cytochrome c oxidase protein 20 homolog Pfam PF12597

Biochemistry

Summary reaction:

4 Fe2+-cytochrome c + 8 H+in + O2 → 4 Fe3+-cytochrome c + 2 H2O + 4 H+out

Two electrons are passed from two cytochrome c's, through the CuA and cytochrome a sites to the cytochrome a3- CuB binuclear center, reducing the metals to the Fe2+ form and Cu+. The hydroxide ligand is protonated and lost as water, creating a void between the metals that is filled by O2. The oxygen is rapidly reduced, with two electrons coming from the Fe2+cytochrome a3, which is converted to the ferryl oxo form (Fe4+=O). The oxygen atom close to CuB picks up one electron from Cu+, and a second electron and a proton from the hydroxyl of Tyr(244), which becomes a tyrosyl radical: The second oxygen is converted to a hydroxide ion by picking up two electrons and a proton. A third electron arising from another cytochrome c is passed through the first two electron carriers to the cytochrome a3- CuB binuclear center, and this electron and two protons convert the tyrosyl radical back to Tyr, and the hydroxide bound to CuB2+ to a water molecule. The fourth electron from another cytochrome c flows through CuA and cytochrome a to the cytochrome a3- CuB binuclear center, reducing the Fe4+=O to Fe3+, with the oxygen atom picking up a proton simultaneously, regenerating this oxygen as a hydroxide ion coordinated in the middle of the cytochrome a3- CuB center as it was at the start of this cycle. The net process is that four reduced cytochrome c's are used, along with 4 protons, to reduce O2 to two water molecules.

Inhibition

Cyanide, sulfide, azide, and carbon monoxide[14] all bind to cytochrome c oxidase, thus competitively inhibiting the protein from functioning, which results in chemical asphyxiation of cells. Methanol in methylated spirits is converted into formic acid, which also inhibits the same oxidase system. High levels of ATP can allosterically inhibit cytochrome c oxidase, binding from within the mitochondrial matrix.[15]

Subcellular distribution

Cytochrome c oxidase has 3 subunits which are encoded by mitochondrial DNA. Of these 3 subunits encoded by mitochondrial DNA, two have been identified in extramitochondrial locations. In pancreatic acinar tissue, these subunits were found in zymogen granules. Additionally, in the anterior pituitary, relatively high amounts of these subunits were found in growth hormone secretory granules.[16] The extramitochondrial function of these cytochrome c oxidase subunits has not yet been characterized. Besides cytochrome c oxidase subunits, extramitochondrial localization has also been observed for large numbers of other mitochondrial proteins.[17][18] This raises the possibility about existence of yet unidentified specific mechanisms for protein translocation from mitochondria to other cellular destinations.[16][18][19]

Clinical significance

Defects involving genetic mutations altering cytochrome c oxidase (COX) functionality or structure can result in severe, often fatal metabolic disorders. Such disorders usually manifest in early childhood and affect predominantly tissues with high energy demands (brain, heart, muscle). Among the many classified mitochondrial diseases, those involving dysfunctional COX assembly are thought to be the most severe.[20]

The vast majority of COX disorders are linked to mutations in nuclear-encoded proteins referred to as assembly factors, or assembly proteins. These assembly factors contribute to COX structure and functionality, and are involved in several essential processes, including transcription and translation of mitochondrion-encoded subunits, processing of preproteins and membrane insertion, and cofactor biosynthesis and incorporation.[21]

Currently, mutations have been identified in seven COX assembly factors: SURF1, SCO1, SCO2, COX10, COX15, COX20, COA5 and LRPPRC. Mutations in these proteins can result in altered functionality of sub-complex assembly, copper transport, or translational regulation. Each gene mutation is associated with the etiology of a specific disease, with some having implications in multiple disorders. Disorders involving dysfunctional COX assembly via gene mutations include Leigh syndrome, cardiomyopathy, leukodystrophy, anemia, and sensorineural deafness.

Histochemistry

COX histochemistry is used for mapping regional brain metabolism in animals, since there is a direct relation between enzyme activity and neuronal activity.[22] Such brain mapping has been accomplished in spontaneous mutant mice with cerebellar disease such as reeler[23] and a transgenic model of Alzheimer's disease.[24] This technique has also been used to map learning activity in animal brain.[25]

Additional images

  • ETC

  • Komplex IV.png

    Complex IV

See also

References

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  24. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  25. ↑ Lua error in package.lua at line 80: module 'strict' not found.

External links

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